1. Field of the Invention
The present invention relates to touch control systems of the type which include at least one touch sensitive device which capacitively transmits a relatively large electrical drive signal when not touched and transmits a relatively small electrical signal when touched and means for detecting the touch and no-touch conditions of the touch sensitive devices. Such touch control systems are commonly used in conjunction with other control circuitry to activate and/or deactivate functions of an apparatus.
Generally speaking, the improved detection means of the present invention utilizes a high gain voltage comparator having positive feedback associated therewith which allows the multiplexing of a plurality of touch sensitive devices to such detection means at low drive voltages and which may be operable either asynchronously or synchronously with other control circuitry.
2. Description of the Prior Art
In substantially all touch control systems it is necessary that the touch and no-touch conditions of touch sensitive devices be distinguishable and detectable to control a desired function. Accordingly, the detection means must distinguish between the voltage amplitude of an electrical drive signal where there is a no-touch condition and the voltage amplitude where there is a touch condition. It is therefore desirable that the detection means include devices with precisely defined thresholds which will clearly and discretely detect the difference between large and small electrical signals. Furthermore, it would be desirable that the detection means have a high voltage gain so that low voltage drive signals may be utilized and a plurality of touch sensitive devices may be multiplexed to a single detection means without requiring a significantly high drive voltage.
Previous touch control systems have utilized P-channel metal oxide semiconductors (PMOS) and complementary metal oxide semiconductors (CMOS) as threshold detection devices (See Walter R. Spofford's copending application, Ser. No. 762,779 entitled "An Implemental Means For A Touch Control System"). However, PMOS devices do not exhibit clean threshold characteristics, they have a low voltage gain, and in general they vary widely from device to device making reproducibility of such devices on a large scale with uniform characteristics very difficult. Accordingly, PMOS devices when utilized in touch control systems perform inadequately and inefficiently and thereby result in decreased reliability and increased cost of the overall touch control system. Furthermore, because of the low voltage gain, very high drive voltages are required when multiplexing a plurality of touch sensitive devices to a single detection means including PMOS devices.
As disclosed by Walter R. Spofford in his aforementioned copending application, by utilizing multiple input inverting logic gates which include CMOS devices three touch sensitive devices may be detected utilizing a detection means which is a single integrated circuit package. The integrated circuit package is capable of being manufactured such that each of the CMOS devices exhibits uniform characteristics and therefore at least for the detection of three touch sensitive devices the aforementioned problems associated with PMOS devices are overcome. However, CMOS devices exhibit low voltage gain, similar to PMOS devices, which limits their use in a multiplexed detection scheme.
When adapting a touch control system to a larger control system requiring asynchronous operation the detected touch condition must be remembered. When adapting a touch control system to a larger control system requiring synchronous operation the proper detector state, i.e. touch or no-touch, must be maintained for some minimum pulse width. For either or both of these operating conditions, MOS devices require additional circuitry and/or the imposition of greater restrictions on their thresholds to make them adaptable to the desired mode of operation.